1. Field of the Invention
The present invention relates to a method and system for correcting extension rates of images.
2. Description of the Related Art
In the digital radiography system, there has been employed an energy subtraction process. In the process, two images formed by different energies are subtracted each other, and the difference thus obtained is visually presented. Two methods, a one-shot method and a two-shot method, have been used for taking X-ray images in use for the energy subtraction process.
In the two-shot method, an X-ray tube is set at a low voltage, and an object is irradiated with the X-rays emitted from the X-ray tube. Then, the X-ray tube is set at a high voltage, and the object is irradiated with the X-rays emitted from the X-ray tube. By the above process, two images by different energies are obtained.
In the one-shot method, a pair of imaging plates (IPs) sandwiching an X-ray absorption material such as cooper are used. The X-ray tube radiates one shot of X-rays to an object. Accordingly, as existence of the x-ray absorption material between the IPs causes a difference between the energies radiated to the IPs, two images by different energies are obtained.
By the above two methods, an image of the soft part tissue except for the bone or an image of the bone except for the soft part tissue can be obtained. The larger the energy difference between two images is, the easier the subtraction process is. To perfectly separate the bone from the soft part tissue, therefore, the two-shot method is superior to the one-shot method.
Apparatuses for switching the X-ray tube voltage at a high speed and moving the two IPs at a high speed to obtain two images, are necessary to the two-shot method. When an object respires, a motion of the object causes a position shift between two images.
In the one-shot method, an energy difference between the two images is small. However, as the images are formed on the basis of one-shot radiation of X-rays, the position shift between the images does not occur. Further, switching apparatus and IP moving apparatus are not required.
To apply the subtraction process to the two images, the image position fitting process is performed by moving and rotating one image with respect to the other image.
In the two-shot method, a distance between the X-ray tube and the IP is constant for the two-shot radiations. Accordingly, the two images are equal in size, or extension rate. Therefore, the images can be position-fit by the above fitting process.
In the one-shot method, a distance between one IP and an object is different from that between the other IP and the object by the thickness of the X-ray absorption material. Accordingly, the extension rate of one image is different from that of the other image. For this reason, the images cannot be position-fit by only the above fitting process. If the subtraction process is applied to the images having different extension rates, a contour of the tissue, which is to be removed is left according to the extension rate, thereby reducing a resolution of the image.
It is demanded that an apparatus capable of improving the resolution of the images by correcting a position shift in accordance with different extension rates of the images.